Open Shortest Path First (OSPF) is a routing protocol developed for Internet Protocol (IP) networks based on the shortest path first or link-state algorithm.
Routers use link-state algorithms to send routing information to all nodes in an IP network by calculating the shortest path to each node based on a topography of the IP network constructed by each node. Each router maintains a routing table in which the router stores routes to particular network destinations. Each router sends the state of its own links to other routers in the IP network.
The advantage of shortest path first algorithms, compared to distance vector algorithms, is that they result in smaller more frequent updates everywhere in the IP network. They also converge quickly, so that problems such as routing loops and infinite counts, in which routers continuously increment the hop count to a particular destination node, are avoided.
A disadvantage of OSPF algorithms is that they require a great deal of processing power and memory in the OSPF routers. However, it is generally believed that the advantages outweigh the disadvantages, and OSPF Version 2, which is defined by RFC 1583, appears to be rapidly replacing Reservation Initiation Protocol (RIP) on IP networks.
Current OSPF link-state routing protocols flood Link-State Advertisements (LSAs) to all OSPF routers. This provides OSPF routers with routing information for routing payload packets on a hop-by-hop basis. However, for the purposes of establishing paths for Traffic Engineering (TE), TE routing information is only required at a point where the path setup is triggered. Consequently, it is not necessary for all routers in a routing area to flood, process and store a large number of TE-LSAs.
If each OSPF router in an IP network of a number (n) of OSPF routers originates a number (L) of LSAs and floods the LSAs on a number (p) of ports, each router in the network potentially receives up to n*L*p LSAs. The total number of LSAs processed by all OSPF routers in the network is therefore potentially n*n*L*p LSAs. This places an unnecessarily large computation load on the OSPF routers, and forces the provision of high speed processors and a large amount of memory in the routers. It also appropriates network bandwidth for LSA exchange that could be more profitably used for payload packet traffic.
FIG. 1 is a schematic diagram of an Internet Protocol (IP) network 10 that uses an Open Shortest Path First (OSPF) routing protocol in accordance with the prior art. The network also includes a plurality of routers 12–22, labeled as routers A–G. The structure and functions of the network 10 are well known in the art.